Current drive efficiency of long-duration and high-density discharge in TRIAM-1M

Experiments of non-inductive current drive plasma have been carried out by using high frequency (8.2 GHz) lower hybrid waves in the superconducting tokamak TRIAM-1M (R=0.84 m). Fairly high density plasma (ne>1019 m-3) has been maintained for 23 s. In shorter pulse LHCD discharges, current drive efficiency has been conventionally estimated as ηCD = neIpR/PRF. By using this figure of merit ηCD, its dependences on machine and plasma parameters (i.e. ΔΦ, Bt and ne) at the initial stage of the discharge are examined. However, it is uncertain that ηCD is an available figure of merit for the extrapolation to the plasma sustained for a long time. Then, temporal evolutions of the plasma parameters are measured and the current drive figure of merit during the long duration (≥ 10 s) discharge is studied. The plasma current and the line averaged electron density are maintained constant for about 4 s. Nevertheless, they begin to decrease and ηCD decreases accordingly. The cause of the deterioration of ηCD and the correlation between ηCD and the confinement of the bulk electrons are investigated. (author) 5 refs., 4 figs., 1 tab